The requirements of the development of mobile communication are supporting higher transmission rate, more perfect signal coverage and higher resource utilization ratio. The relay technology can improve coverage and balance, and increases cell throughput, and the relay station has a lower configuration cost as compared with the base station, therefore, the relay is considered as a crucial technique in the an evolved Long Term Evolution (LTE) system-LTE-Advanced (LTE-A) system.
In the Time Division Dual (TDD) LTE system, frequency resources are divided by using frame as a unit in terms of time. As shown in FIG. 1, the length of each wireless frame is 10 milliseconds, each wireless frame comprises 10 subframes with a length of 1 millisecond, comprising uplink subframes, downlink subframes and special subframes respectively.
TABLE 1Uplink and downlink configuration forms ofsubframes in a TDD LTE systemUplink and downlinkNumber of subframeconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
Depending on different ratios of uplink subframes to downlink subframes, there are totally seven types of subframe configuration structures in the LTE TDD systems, the ratio of uplink subframes to downlink subframes in each configuration structure is different, and the system can configure flexibly according to the service volume in the uplink and downlink in the cell. The specific seven configurations are as shown in Table 1, wherein D denotes a downlink subframe, U denotes an uplink subframe, and S denotes a special subframe. The structure of S subframe is as shown in FIG. 2, comprising a downlink pilot frequency time slot (DwPTS), a gap of protection (GP), and an uplink pilot time slot (UpPTS). As an evolved system of TDD LTE for ensuring backward compatibility, the TDD LTE-A system will retain the same frame structure as the TDD LTE.
HARQ (Hybrid-Automatic Repeat Request) is an important error control method in the packet transmission system, and can efficiently improve transmission reliability. In the LTE/LTE-A system, uplink HARQ transmission defines a series of transmission timing sequences, including: uplink data transmission and downlink Acknowledge/Negative Acknowledge (ACK/NACK) information feedback timing sequences; downlink ACK/NACK feedback and uplink data retransmission timing sequences, etc. Wherein, in the LTE/LTE-A system, uplink data transmission is on the subframe #n, and the corresponding downlink Acknowledge/Negative Acknowledge (ACK/NACK) information feedback is on the subframe #(n+kPHICH), wherein the value of kPHICH is determined by the uplink and downlink subframe configuration in the seven types of TDD, as shown in Table 2.
TABLE 2Value of kPHICH in TDDTDD uplink andUplink subframe index ndownlink configuraiton0123456789047647614646266366646656646647
As shown in FIG. 3, it shows the architecture of the mobile communication system incorporating the Relay Node (RN). In the mobile communication system, the link between an evolution Node B (eNB) and a relay node is called as a Backhaul Link (also called as a Un Link), the link between a RN and a User Equipment (UE) within its coverage range is called as an Access Link (also called as a Uu Link), and the link between the eNB and the UE within its coverage range is called as a Direct Link. For the eNB, the RN is equivalent to a UE; for the UE, the RN is equivalent to an eNB.
Currently, when the in-band relay mode is adopted, i.e., the Un link and Uu link use the same frequency band, the RN cannot implement transmitting and receiving operations simultaneously on the same frequency resource in order to avoid transmitting and receiving interference of the RN itself. That is, when the RN transmits downlink data to its subordinate UE, it cannot receive downlink data from the eNode-B, or when the RN receives the down link data from the eNode-B, it cannot transmit data to its subordinate UE; similarly, when the RN receives uplink data from its subordinate UE, it cannot transmit uplink data to the base station, or when the RN transmits uplink data to the base station, it cannot receive uplink data from its subordinate UE.
In this case, it needs to set aside a part of the uplink and downlink resources respectively for the eNB-RN communication resource (the corresponding interface is also called as an Un interface), that is, part of subframes are set aside for eNB-RN communication, and these subframes are called as relay subframes (or Un subframes).
For the downlink relay subframe, the relay station indicates to the subordinate Rel-8 UE that it is a MBSFN subframe, and transmits downlink control information to its subordinate UE only in the control domain of the MBSFN subframe, but does not perform any transmission to its subordinate UE in the resources outside the control domain of the MBSFN subframe, thereby ensuring the compatibility of the relay station with the Rel-8 UE when performing downlink receiving, as shown in FIG. 4.
For the uplink relay subframe, the uplink service of the subordinate UE is not scheduled to avoid the UE from performing uplink transmission at the mean time of uplink transmission of the RN. However, in order not to affect the data transmission of the subordinate UE of the RN and to ensure the reliability of transmission between the eNB and RN, it needs to define the uplink HARQ timing sequence of the relay link.
At present, in the 3GPP conference discussion, the discussion on the uplink and downlink HARQ timing sequences of the Un link is very hot, but no consistent view is reached yet.